1. Technical Field of the Invention
The invention relates generally to communication systems; and, more particularly, it relates to a system and method that are operable to provide for improved communication channel estimation using information provided by redundant training tones (TTs)/pilot tones.
2. Description of Related Art
Communication systems transmit digital data through imperfect communication channels. These symbols may undergo some undesirable corruption due to the imperfection of the communication channel. One effort to try to avoid such situations is focused on performing forward error correction (FEC) coding. However, there is typically some difficulty in extracting the information contained within these symbols after they have been undesirably altered within the communication channel. There exist some methods that seek to curb the effect that the communication channel has had on the data; one such method includes employing using Decision Feedback Equalizers (DFEs). However, even after the incoming signal has been equalized, the extraction of the data, that has undergone some alteration due to the channel effects, is still a probabilistic determination. Many communication systems seek to estimate the actual channel response of a communication channel. Of the many deficiencies that may be introduced in doing this, one major problem is the undesirable introduction of noise into both the estimate of the channel response as well as the estimate of the channel noise within the communication system.
A current implementation of the Vector Orthogonal Frequency Division Multiplexing (VOFDM) standard employs a methodology for characterizing the channel response according to the following: pilot tones or training tones (PTs or TTs) are sampled during multiple sampling periods that are separated in time. Each sampling period corresponds to a block of symbols that has been modulated using the VOFDM method of translating the symbols into the transmission waveform via an FFT operation. Each such block of symbols and their resulting FFT-generated waveform will henceforth be termed a frame. In general, a transmission burst consists of multiple frames. TTs are then sampled for each of the frames in a burst transmission. The TTs from a single frame can be processed to produce an estimate of the actual communication channel's channel response embedded in noise. A noise channel estimate may be referred to as a noisy channel response. The TT samples from a multiplicity of frames are processed to produce the estimate of the channel response. Generally speaking, the average of the multiplicity of noisy channel responses is considered to be the channel response, while the difference between noisy channel responses and this average is considered to be noise. Noise estimates are necessary in the VOFDM processing, as well as within processing a channel response estimate.
As an example, an embodiment employing two frames in a transmission burst is considered. The two TT sample groups (TTs in frame one, and TTs in frame two) are processed to produce their noisy channel responses, and these two noisy channel responses are averaged to produce the channel response. The difference between the two TT samples' noisy channel responses is considered to be noise of the communication channel.
While this prior art methodology may appear to work satisfactorily for very slowly time varying channels or for non-time varying channels (static communication channels), the prior art methodology simply fails to produce a channel response that adequately tracks rapidly changing channels. Also, this methodology requires a minimum of two upstream frames per upstream transmission burst, unless it is otherwise guaranteed that multiple grants, closely spaced in time, are provided for each user.
Further limitations and disadvantages of conventional and traditional systems will become apparent to one of skill in the art through comparison of such systems with the invention as set forth in the remainder of the present application with reference to the drawings.